Not-So-Muddy Waters Put Millions at Risk

Wed, 09/23/2009 - 5:30pm

More evidence that dams really are a dirty business

Muddy waters of the Tekeze River, Ethiopia

The world could see an epidemic of “Hurricane Katrina” destruction from storms if dam builders persist in bottling up more rivers. Most of the world’s major river deltas are sinking, thanks in large part to dams withholding land-building sediments, a new scientific study reveals. The authors estimate that the subsidence is increasing flood risk for half a billion people.

Hundreds of scientists from dozens of federal labs and universities around the US were involved in the study, which looked at 33 major deltas (24 of which were found to be sinking).

A BBC report says the scientists calculate that "85% of major deltas have seen severe flooding in recent years, and that the area of land vulnerable to flooding will increase by about 50% in the next 40 years as land sinks and climate change causes sea levels to rise.”

MSNBC reports: “The flooding will increase even more if the capture of sediments upstream from deltas by reservoirs and other water diversion projects persists and prevents the growth and buffering of the deltas, according to the study.”

I work to protect the rivers of Africa. Many of the continent’s river deltas have already suffered from a loss of silt. Two of Africa’s higher-risk rivers for subsiding deltas are the Nile and Zambezi, the continent’s second and fourth largest rivers. Both have already suffered dramatic silt-deprivation from large dams, with more dams on the way to “seal the deal”.

Before the Aswan Dam was built, the Nile’s annual floods deposited 4 million tons of nutrient-rich sediment in Eypt, supporting agriculture and delta fisheries. Instead of supporting a rich (and protective) delta, the sediments are now filling the reservoir and decreasing its storage capacity. Downstream, farmers are forced to use about a million tons of artificial fertilizer a year as a substitute for the naturally deposited nutrients from the annual flood.

Today, the Sudanese government has completed the destructive Merowe Dam on the Nile, a project that has forced thousands of farmers off their riverine farms into harsh desert resettlement camps. Upstream, Ethiopia is building a handful of dams on its highly silty Nile tributaries, including the Tekeze Dam, which experts believe will silt up so fast as to make the project quickly unviable.

Tekeze Dam, Ethiopia

The Zambezi’s major dams have also led to a shrinking delta. Scientists report that some 11,322 acres of wetlands and coastal barriers have been lost between 1986 and 2000 – an average annual rate of wetland loss of 424 acres a year.

When the Cahora Bassa Dam was built in 1973, its managers decided to fill it in one flood season, going against recommendations to fill over at least two years. This huge drop in river flow led to a 40% reduction in the coverage of mangroves, greatly increased erosion of the coastal region and a 60% reduction in the catch of prawns off the mouth due to the reduction in nutrient-rich silt.

Mozambique appears to be getting hit with more and stronger tropical storms in recent years, making those protective wetlands that much more important. Yet the nation wants to build another large dam, Mphanda Nkuwa, downstream of the huge Cahora Bassa dam.

Healthy rivers are important for so many reasons, and climate change will only make their gifts that much more important. It’s certainly harder to argue to “protect the silt” than it is to, say, Free Willy or save the whales – but perhaps that is what we need to do to get it through the heads of those now arguing that climate change calls for more hydropower. Perhaps we could form a radical river-silt protection movement: "Dirt First! And dams last."